An emulsion is a mixture of two immiscible liquids, where one liquid is suspended in the other in the form of small droplets. The term immiscible denotes the presence of an energetic barrier to creation of an interface. There is no co-dissolution of the separate phases. The energetic barrier is manifest as interfacial tension, γ, between the two liquids. The Gibbs Free Energy, G, of the system increases with interface formation, δσ as expressed in Equation 1 below.δG=−SδT+Vδp+γδσ  (Eqn. 1)                where δσ is the change in surface areaAn emulsion is formed when energy is applied to the system. Energy sources include mixing, pumping, heating, and fluid transfer. Input energy allows drops to rupture and the surface area of the liquid-liquid interface to increase from its smallest size, a single surface between two bulk layers, to a much larger size, a multitude of surfaces between drops of one liquid suspended in a continuous phase of the other liquid. The higher the energy input, the higher the surface area of emulsified drops, and the lower the drop size.        
An emulsion is a high energy state, and as such, without continuous energy input, will relax to the lowest surface area configuration of two separate bulk phases separated by a single interface. For an emulsion to relax, drops must encounter one another, collide, and coalesce to a larger drop. This process is kinetic and its speed is subject to factors that alter the energy barrier to coalescence.
The need to separate emulsions of water and hydrocarbons is ubiquitous; historically impacting a broad array of industries. Prior art for separation of water-hydrocarbon emulsions includes systems that rely on single or multiple elements, novel flow patterns, stilling chambers, parallel metallic plates, oriented yarns, gas intrusion mechanisms, and electrostatic charge. The balance of separation systems employ an element that contains a fibrous, porous coalescing media through which the emulsion is passed and separated. Irrespective of the system design, all water-hydrocarbon separation systems target the collection of emulsified drops into close proximity to facilitate coalescence. Coalescence and subsequent separation due to density differences between water and hydrocarbons is mechanism behind all separation systems.
Prior art fibrous, porous coalescence media induce emulsion separation in flow-through applications through the same general mechanism, irrespective of the nature of the emulsion. The coalescence media presents to the emulsion discontinuous phase an energetically dissimilar surface from the continuous phase. As such, the media surface serves to compete with the continuous phase of the emulsion for the discontinuous, or droplet, phase of the emulsion. As the emulsion comes in contact with and progresses through the coalescing media, droplets partition between the solid surface and the continuous phase. Droplets adsorbed onto the solid media surface travel along fiber surfaces, and in some cases, wet the fiber surface. As more emulsion flows through the media, the adsorbed discontinuous phase encounters other media-associated droplets and the two coalesce. The drop migration-coalescence process continues as the emulsion moves through the media. A coalescence media is successful for breaking a given emulsion if the discontinuous phase preferentially adsorbs or is repelled and, at the point of exit from the media, the droplet phase has been coalesced to sufficiently large drops. The drops separate from the continuous phase as a function of density differences between the liquids involved. A coalescence media is unsuccessful for breaking an emulsion if, at the point of exit from the media, the drops remain sufficiently small that they remain entrained by the continuous phase and fail to separate.
Media of the prior art have employed fibers with surface energy matched to the discontinuous phase. Hydrophilic fibers such as glass or nylon are used to coalesce water droplets out of hydrocarbon continuous phases. Hydrophobic fibers such as styrene or urethane have found use for coalescence of hydrocarbon droplets out of water. Glass and metal have been used to coalesce oil while polyester and PTFE have been used to coalesce water. In these cases, formation of multiple interfaces between the discontinuous phase and the solid is required. This is an energetically unfavorable state for the droplet phase. Due to this, drops adopt the lowest energy configuration and collects on the media surface. As more emulsion flows through the media, more drops collect.
Examples of the prior art include U.S. Pat. No. 3,951,814 to Krueger which discloses a gravity separator with media in the form of wound sheets or stacked disks consisting of fibers of glass, ethylene, propylene, or styrene. U.S. Pat. No. 6,569,330 to Sprenger and Gish discloses a filter coalescer cartridge consisting of two layers of pleated media disposed in a concentric nest and consisting of fiberglass that may contain two differing diameters. U.S. Pat. No. 6,332,987 to Whitney et al. discloses a coalescing element that incorporates porous structures that involve a wrap consisting of polyester. U.S. Pat. Nos. 5,454,945 and 5,750,024 to Spearman disclose a conical coalescing filter element consisting of pleated, flat media of randomly oriented fibers of glass, polymer, ceramic, cellulose, metal, or metal alloys. U.S. Pat. No. 4,199,447 to Chambers and Walker discloses coalescence of oil in oil-water emulsions by passing the emulsion through a fibrous structure with finely divided silane coated silica particles adhered to their surfaces. U.S. Pat. No. 4,199,447 to Kuepper and Chapler discloses a waste water oil coalescer apparatus with tubular coalescer elements consisting of oleophilic fabric, cotton, polypropylene, and fabric woven from natural and synthetic fibers that may include metallic threads. U.S. Pat. No. 5,997,739 to Clausen and Duncan discloses a fuel/water separator that contains an element consisting of coalescing media that is a flexible sock, a nylon mesh, or cloth media. U.S. Pat. No. 5,993,675 to Hagerthy discloses a fuel-water separator for marine and diesel engines that contains a microfibrous filter element constructed of various types of polymer fibers.
Other examples of the prior art include U.S. Pat. No. 5,928,414 to Wnenchak et al. which discloses a cleanable filter media made up of expanded PTFE layers as well as spunbonded polyester and nonwoven aramid felt. U.S. Pat. No. 4,588,500 to Sprenger and Knight discloses a fuel dehydrator designed for fuel-shut-off that has layers of cellulose and fiberglass sheets wound around a porous tube. U.S. Pat. No. 4,372,847 to Lewis discloses an assembly to remove contaminants from fluid that includes a demulsifier cartridge containing pleated hydrophobic treated cellulose media or fiberglass. U.S. Pat. No. 5,225,084 to Assmann discloses a process for the separation of two immiscible organic components using a fibrous bed consisting of glass fibers or a mixture of glass and metal fibers. U.S. Pat. No. 5,417,848 to Erdmannsdorfer et al. discloses a coalescence separator with a changeable coalescence element containing microfine fiber material. U.S. Pat. No. 6,422,396 to Li et al. discloses a coalescer design for hydrocarbons containing surfactant comprised of at least three layers of polymeric hydrophobic media including polypropylene and polyester. U.S. Pat. No. 6,042,722 to Lenz discloses a single separator for removal of water from various fuels, including diesel and jet fuel. U.S. Pat. Nos. 6,203,698 and 5,916,442 to Goodrich disclose the use of hydrophobic filter media to reject water on the upstream side of the filter. U.S. Pat. No. 5,993,675 to Hagerthy discloses the use of entangled microfibers, which are impervious to the passage of water, but which allow the fuel to flow through. U.S. Pat. No. 7,285,209 to Yu et al. discloses an apparatus for removing emulsified water from surfactant containing hydrocarbons using a first filter to strip surfactants from the hydrocarbon made of nylon, polyester, polyvinylidene difluoride, or polypropylene, and a second cross-flow filter in spiral wound cartridges, tubular cartridges, or hollow fiber cartridges made of polytetrafluoroethylene membrane.
From the above, it is clear that innovation in the coalescence and separation arena often involves complete separation systems. The systems involve multiple media types, multiple media elements, and multiple layers of media. The innovation often concerns packaging the media and flowing the emulsion in novel ways. The drawback of this approach is complexity, which translates directly to manufacturing and raw material costs. The same factors that give rise to complexity and increased cost, also limit universal applicability of the solution. New solutions invariably concern a single or extremely limited set of coalescing filter designs. Missing from the prior art is a single-roll media capable of water separation from hydrocarbons that is universally compatible with separation systems already in use and commodity converting technologies.
In addition, hydrocarbons, particularly diesel fuels, increasingly are dosed with surfactants. The surfactants come in the form of fuel additives such as lubricity enhancers and rust inhibitors, as well as biodiesel. Biodiesel is a blend of fatty acid methyl esters derived from methanol esterification of plant and animal triglycerides. Escalating oil prices as well as pressure for domestic fuel supply development and minimization of fossilized carbon emission create conducive conditions for biodiesel substitution for hydrocarbons in various transportation, power generation, and industrial applications. Biodiesel was also found to improve diesel fuel lubricity, and as a result generated additional impetus for its use as a blend component for low lubricity Ultra Low Sulfur Diesel fuel. Such blends of hydrocarbons and surfactants create conditions where systems designed in the past for water removal from hydrocarbons fail and allow 50-100% of entrained water to pass uninhibited through the separation system into the end use.
Surfactants promote the formation of smaller drops within emulsions and stabilize emulsions against separation. Surfactant is an abbreviation for the term “surface active agent.” Surfactants are molecules that contain two parts, one known as lyophilic, or solvent liking, the other as lyophobic, or solvent hating. In instances where the solvent phase is water, the terms become hydrophilic and hydrophobic. In the case of an emulsion, the solvent would be the continuous phase. This housing of dual affinities in one molecule imparts to surfactants their surface active properties. In order to minimize energy, surfactants align at interfaces to allow both parts of the molecule to reside in a favorable environment. The presence of a surfactant at the interface of two immiscible liquids lowers the interfacial tension, and as a result, lowers the energy required for drop rupture to form an emulsion (Eqn. 1). In the presence of a solid-liquid interface, the lyophobic group of the molecule aligns on the solid, and the lyophilic extends away from the surface. Surfactants within an emulsion populate liquid-liquid interfaces, as well. In this case, however, there are hundreds of square meters of interface surface generated by the droplet phase. In an emulsion, surfactants align the lyophobic moiety toward the droplet, and extend the lyophilic group outward into the continuous phase. This creates conditions where the drops are insulated both from the continuous phase by the lyophobic group and, through interaction of lyophilic groups, from other drops. Both of these factors place an energetic barrier to the relaxation of the emulsion to its lowest energy state of two separate bulk phases. The schematics in FIG. 1 illustrate surfactant interactions that lead to emulsion stabilization.
The surfactant properties discussed above lead to failure of prior art media and prior art water separator systems designed for separation of water-hydrocarbon emulsions. Irrespective of separator design, all water-hydrocarbon separators function by providing a solid surface upon which the emulsion discontinuous phase is destabilized, the discontinuous phase is coalesced, and is allowed to gravimetrically settle out of the continuous phase. In order to be destabilized, the droplets of the discontinuous phase must contact the solid surface. By lowering the energy of drop rupture, emulsion droplet sizes in the presence of surfactants are considerably smaller. This creates conditions where the discontinuous phase drop size is sufficiently small to pass through the media with minimal contact with the media surface, thus avoiding the surface generated destabilization key to successful to coalescence and separation. Further, by stabilizing the droplets within the continuous phase, surfactants interfere with the natural adsorption of the discontinuous phase on the media surface. The media surface must successfully compete for components of an emulsion. By stabilizing droplets in the continuous phase, surfactants lower the energy of the emulsion and lower the probability that the droplet phase will preferentially adsorb to the media surface. Finally, through adsorption to droplet surfaces, surfactants change the surface characteristics of the discontinuous phase. Emulsion destabilization by the solid surface is effected through its surface energy. Through adsorption to solid surfaces, surfactants alter media surfaces and thus dramatically change the nature of the interaction between the surface and the discontinuous phase. The result is a mass homogenization of system energies that fully disarm the capability of prior art emulsion separation media and prior art emulsion separation systems.
Coalescence is a liquid-solid or adsorption based separation. For separation to occur, the phases to be separated must interact with the solid surface. Partitioning of emulsion components between the solid media surface and the hydrocarbon is driven by free energy minimization (Eqn. 1). An emulsion component will associate with the solid media if that interaction lowers the overall energy of the system. At constant temperature and pressure, energy minimization will be driven by the □□□ term. Components with a low solid-liquid interfacial tension (high affinity for solid) will exhibit a higher surface of interaction compared to components with high solid-liquid interfacial tension (low affinity for solid). Surface of interaction or surface area, translates to path length available for that component's journey through the stationary phase. Path length drives elution time from the media. Elution time determines effectiveness of separation. In difficult to resolve mixtures, where only minimal differences exist between phases to be separated, elution time differences are exaggerated by increasing the available path length. Relative to emulsion separation, surfactants interact with the solid, as well, and can be stripped from droplets through preferential adsorption to the solid. This process also promotes coalescence, as droplets are destabilized and prone to coalescence in the absence of surfactant. The greater the surface available for adsorption, the higher the probability of interaction and successful emulsion destabilization. As a result, in adsorption-based separation, surface area of the stationary phase, is the single most critical parameter for successful separation.
Prior art coalescence media fail to separate emulsions when the solid-liquid interaction fundamental to dispersed phase destabilization is interrupted. As such, failure of prior art media derives from failure to achieve sufficient interaction with the media surface. This failure occurs through two pathways, inappropriate pore size and insufficient surface area. With regard to pore size, droplets of water emulsified in surfactant containing diesel fuels with low interfacial tension were found to fall in the 3.5 micron range, a dramatic shift from the 10.0 micron range typical of diesel fuel and kerosene without surfactants. Prior art media are not designed to capture droplets of this small size. As a result, when the droplet phase consists of drops sufficiently small in size to escape through the media with minimal interaction with media surfaces, the droplets are not coalesced and the prior art coalescing media fails.
By homogenizing system energies, surfactants also render insufficient the surface area of prior art coalescing media. In surfactant stabilized emulsions, interference exists between the droplet phase and the media surface due to surfactant adsorption at liquid-liquid and solid-liquid interfaces. As described above, surfactant adsorption on interfaces equalize energies of interaction and require longer path length for effective resolution of emulsion components. Surface areas of prior art media are simply not large enough to provide the required path length for separation. Due to this, when presented with a surfactant stabilized water-hydrocarbon emulsion, prior art media are easily overwhelmed and altered by adsorbed surfactants, and the discontinuous phase passes through the media uncoalesced. Failure of common coalescence media currently occurs in surfactant and biodiesel-containing fuels.
The need to successfully interact with discontinuous phase droplet sizes below 5.0 microns in diameter and the need to dramatically increase surface area place media characteristics in conflict with end use needs such as permeability and thickness. Flow rate requirements are the fundamental driver of permeability and thickness targets. Separation is invariably promoted if the velocity through the media is slowed to give maximum contact time with the surface. This of course can not be accommodated by the end use which stipulates minimum operating flows through the media. Minimum flow requirements, in turn, drive permeability targets for maintenance of practical pressure drops over the media. Flow requirements dictate velocities through the media. Velocities are a derivative of the area of media used for a given separation. As elements employ pleated or wound media, media thickness determines the area of media that can be used in a given application, and as such, the velocity of the emulsion through the media. Separation is promoted by media that can effect separation at the lowest thickness, or caliper.
With regard to pore size, pores of prior art media are often too open to force interactions between droplets and the media surface and droplets escape uncoalesced. This occurs when surfactants in the emulsion lower interfacial tension and promote drop rupture to smaller particle size distributions. Prior art media lack pore sizes capable of managing smaller particle size distributions and invariably pass uncoalesced discontinuous phase rto the accepts side of the media. Hypothetically, to effectively interact with small particles, prior art media permeability would need to drop to impractical levels at face velocities required by the end use.
In the case of insufficient surface area, thickness of prior art media place emulsion separation in conflict with velocity requirements. The limiting factor is packing the needed surface area into a sheet or layered sheet of media that has a realistic thickness and, once again, the required permeability. This is not possible with prior art media. The media of prior art are often thick, such as glass mat with a caliper in the range of 5 mm, and require dimensional support, such as wire mesh or a phenolic-resin saturated cellulose sheet. Hypothetically, if prior art media were made to contain sufficient surface area to resolve a complex emulsion such as a surfactant stabilized water-hydrocarbon emulsion, the thickness would be so great, only small amounts would be packable into the separation system housing. Such a limited amount of media would dramatically increase velocity through the media, inhibiting effective separation.
Due to limitations of prior art media, innovation in the coalescence arena often involves “systems” and not media. The systems involve multiple media types, multiple media elements, and multiple layers of media. Systems typically concern packaging the media and flowing the emulsion in various ways to work within the limitations of the pore size—permeability and surface area—thickness, permeability trade-off.